1. Field of the Invention
The present invention relates generally to polyphase filters, and more specifically to polyphase networks for generating output signals shifted in phase.
2. Discussion of the Related Art
In radio communications, polyphase networks are often used to ensure good image rejection in a mixer. As is understood, a polyphase network is a circuit that receives an input signal and produces output signals each having generally the same magnitude as the input signal but shifted in phase. For example, a typical polyphase network produces quadrature outputs shifted by 90 degrees. Polyphase networks conventionally include one or more R-C stages.
A conventional polyphase network is driven by the emitter follower (EF) stage of a bipolar junction transistor (BJT), which provides a low impedance input to the polyphase network. In other words, each input of the polyphase network is at the emitter of a respective transistor. Additionally, the input signal to the polyphase network must be extremely clean otherwise the polyphase network will not provide rejection of undesirable harmonics and spurious content. That is, the 90-degree phase difference at the center frequency can degrade substantially when the input signal is not a pure sine tone. For example, the phase offset may be calculated to be as much as 7 degrees in a non-clean incoming signal or sine wave having a 20 dBc harmonic present.
Furthermore, as is well known, exact quadrature is important to ensure good image rejection in a mixer. For example, a polyphase network producing quadrature outputs that are 4 degrees offset and having a 0.5 dB magnitude imbalance can degrade image rejection to 27 dBc. Depending on the requirements of the system, such performance may be unacceptable.
It is with respect to these and other background information factors that the present invention has evolved.
The present invention advantageously addresses the needs above as well as other needs by providing a polyphase network that provides bandpass shaping to maintain the quadrature phase output.
In one embodiment, the invention can be characterized as a polyphase device comprising a first transistor having a first collector, a first base and a first emitter, the first base adapted to receive a positive portion of an input signal; a second transistor having a second collector, a second base and a second emitter, the second base adapted to receive a negative portion of the input signal, the first transistor and the second transistor adapted to drive the R-C polyphase network; and an R-C polyphase network having a first input coupled to the first collector and a second input coupled to the second collector. The R-C polyphase network includes an inductor and has first output and a second output, the first output and the second output offset in phase. The inductor is adapted to provide bandpass filtering of the first output and the second output.
In another embodiment, the invention can be characterized as a polyphase device comprising a transistor having a collector, a base and an emitter; an R-C polyphase network having an input coupled to the collector, the transistor adapted to drive the R-C polyphase network; and the R-C polyphase network including an inductor adapted to provide bandpass filtering of an output of the R-C polyphase network.
In a further embodiment, the invention may be characterized as a method of generating phase offset signals comprising the steps of: inputting a positive portion of an input signal into a first base of a first transistor; inputting a negative portion of the input signal into a second base of a second transistor; biasing the first transistor and the second transistor to conduct current; driving an R-C polyphase network having a first input coupled to a first collector of the first transistor and a second input coupled to a second collector of a second transistor, wherein the polyphase network includes an inductor; and outputting a first output and a second output, the first output and the second output offset in phase, the inductor causing a bandpass filtering of the first output and the second output.